1. Field of the Invention
The present invention describes a process for removing alcohols from a mixture of thermally sensitive organic phosphorus-containing agents. Hydrogen sulfide is also removed in this process.
2. Introduction to the Invention
It is known from U.S. Pat. No. 4,263,150, issued Apr. 21, 1981 to Clason et al that sulfur-containing salts of phosphorus acids may be prepared. Similar processing to obtain dialkylphosphorodithioic acid salts is found in U.S. Pat. No. 4,308,154 also to Clason issued Dec. 29, 1981. The sulfur-containing salts of dialkylphosphorodithioic acids are typically prepared by contacting a material such as phosphorus pentasulfide with an alcohol corresponding to the alkyl group in the dialkylphosphorodithioic acid. Typically, the alcohol is used in excess and is carried throughout the reaction and eventually recovred following the conversion of the dialkylphosphorodithioic acid to a corresponding metal salt. Hydrogen sulfide is present in the initial reaction mixture as a by-product.
The preparation of the metal salts of the dialkylphosphorodithioic acids involves the generation of water. The neutralization of the dialkylphosphorodithioic acid with, for example, zinc oxide results in the generation of water from the neutralization reaction. Steam treatment of the reaction mixture is also employed to assist in the stripping of excess alcohol. However, the water then contaminates the stripped alcohol. The effect of the steam processing is to further introduce water to the reaction mixture. Thus, the neutralization of the dialkylphosphorodithioic acids and the steam treatment amount to about 5 parts of water per 100 parts of the salt.
The presence of the water, from whatever source, the alcohol, and hydrogen sulfide present various processing difficulties. Any excess alcohol will be difficult to strip from the water. Where azeotropic conditions exist, it will not be possible without undue difficulty to remove the residual alcohol from the water. Conversely, the problem also exists in that some of the water which is bound to the alcohol is likely to remain in the alcohol following stripping. As the largest use of the dialkylphosphorodithioic salts is in lubricants, it is undesirable to have any water carried into the end product. The lower alcohols which are normally water-miscible and processed as described herein may be directly recycled to the reactor to generate more dialkylphosphorodithioic acid. Water wet lower alcohols may not be reused and are not commercially dryable. Excess hydrogen sulfide gas can result in zinc sulfide being formed thereby causing an inorganic salt to be present as well as loss of conversion to the desired zinc salt.
It has been proposed for the recovery of polycyclic alcohols to employ a thin-film evaporator. The foregoing technology is reported in U.S. Pat. No. 4,263,103 to Johnson et al issued Apr. 21, 1981. Similar technology with regard to thermally stabilizing alcohols is found in U.S. Pat. No. 4,198,276 issued June 24, 1977. Dube, in U.S. Pat. No. 4,209,471, issued June 24, 1980, states that a liquid-liquid extraction method is preferable to using a thin film or batch distillation process because of product stability and high vacuum requirements.
Various devices are known for use in thin film evaporation of organic materials. Such equipment is described in a reprinted article entitled "Residence Time Control in Thin-Film Evaporators" by Widmer and Giger which was originally printed in Chemical and Process Engineering, Nov. 19, 1970. A further disclosure of evaporation techniques and equipment is found in an article entitled "Agitated Thin-Film Evaporators" originally published in Chemical Engineering, Sept. 13, 1965.
Ordinarily, the processing of oil soluble components in a thin-film evaporator is conducted on materials which are extremely viscous or even solid at room temperature (20.degree. C.). In the present invention, the dialkylphosphorodithioic acids and excess alcohols which are processed according to the invention are normally liquid materials at temperatures of -40.degree. C.
The present invention deals with the separation of excess alcohol from dialkylphosphorodithioic acids prior to the neutralization of the acid to the corresponding salt. Blowing with dry nitrogen to enhance water removal may be avoided. Water contamination of the alcohol is avoided. Steam treatment to remove higher alcohols is avoided. Hydrogen sulfide gas is separated and may be recovered. The formation of zinc sulfide is avoided herein. Water haze in the end product is also minimized.
Throughout the specification and claims, percentages and ratios are by weight, temperatures are in degrees Celsius and pressures are in KPa gauge unless otherwise noted. Ratios and percentages herein are exemplary and may be combined. To the extent that the references cited herein are relevant to the present invention, they are herein incorporated by reference.